The present invention relates to a process for producing acicular goethite particles which are suitable as a starting material for magnetic particles for magnetic recording, and which have a uniform particle size distribution, are substantially free of dendrites and have a large aspect ratio (major axial diameter/minor axial diameter). The present invention also relates to a process for producing acicular magnetic iron oxide particles which have a uniform particle size distribution, are substantially free of dendrites, and have a large aspect ratio (major axial diameter/minor axial diameter) and an excellent coercive force distribution.
With progressing miniatirization and weight-reduction of magnetic recording and reproducing apparatuses in recent years, the necessity for a recording medium having a higher performance such as a magnetic tape and a magnetic disk has been increasing more and more. In other words, a magnetic recording medium is required to have a higher recording density, higher sensitivity and higher output characteristic. The magnetic characteristics of magnetic particles which are demanded in order to satisfy the above-mentioned requirements for the magnetic recording medium, are a high coercive force and an excellent dispersibility.
In order to improve the sensitivity and the output of a magnetic recording medium, the magnetic particles must have as high a coercive force as possible. This fact is described in, for example, DEVELOPMENT OF MAGNETIC MATERIALS AND TECHNIQUE OF IMPROVING THE DISPERSION PROPERTY OF MAGNETIC POWDER (1982), published by K.K. Sogo Gijutsu Kaihatsu Center, p. 310: "Since the improvement of magnetic tapes has been directed toward a higher sensitivity and a higher output, it is an important point to enhance the coercive force of acicular .gamma.-Fe.sub.2 O.sub.3 particles, . . . "
In order to improve the recording density of a magnetic recording medium, the magnetic recording medium must have a high coercive force and a large residual magnetization (Br), as described in the said DEVELOPMENT OF MAGNETIC MATERIALS AND TECHNIQUE OF IMPROVING THE DISPERSION PROPERTY OF MAGNETIC POWDER. p. 312: "The condition for high-density recording in a coating-type tape is that it is possible to keep the high output characteristics with respect to a short-wavelength signal at a low noise level. For this purpose, it is necessary that both the coercive force (Hc) and the residual magnetization (Br) are large, and that the thickness of the coating film is thin". It is therefore necessary that the magnetic particles have a high coercive force and they are excellent in dispersibility in the vehicle, and orientation property and packing density in the coating film.
In order to enhance the output of a magnetic recording medium, it is required to have a small switching field distribution (hereinunder referred to as "S.F.D.") and hence, the magnetic particles are required to have a small width of coercive force distribution. This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 63-26821 (1988): "FIG. 1 is a graph showing the relationship between the S.F.D. of the above-described magnetic disk and the recording and reproducing output. . . . The relationship between the S.F.D. and the recording and reproducing output is linear, as is clear from FIG. 1. It indicates that the recording and reproducing output is enhanced by using ferromagnetic powder having a small S.F.D. That is, in order to obtain a large output, an S.F.D. of not more than 0.6 is necessary.
As well known, the coercive force of magnetic iron oxide particles depend upon the configurational anisotropy, crystalline anisotropy, strain anisotropy, exchange anisotropy, or the interaction thereof.
Acicular magnetite particles and acicular maghemite particles which are used as magnetic iron oxide particles at present show a relatively high coercive force by utilizing the anisotropy derived from their shapes, namely, by increasing the aspect ratio (major axial diameter/minor axial diameter).
The known acicular magnetic particles are obtained by reducing as a starting material goethite particles or hematite particles obtained by heat-treating the goethite particles, in a reducing gas such as hydrogen to form magnetite particles or iron based metal particles or by further oxidizing the thus-obtained magnetite particles in air to form maghemite particles.
The known acicular magnetic iron oxide particles modified with Co or Co and Fe.sup.2+ are obtained by dispersing acicular magnetite particles or acicular maghemite particles as the precursor particles in an alkaline suspension containing cobalt hydroxide or an alkaline suspension containing cobalt hydroxide and ferrous hydroxide so that 0.1 to 21.0 atomic % of Co based on Fe of the precursor particles is contained, and heat-treating the resultant dispersion.
The residual magnetization (Br) in a magnetic recording medium depends upon the dispersibility of the magnetic particles in the vehicle, and the orientation property and packing density of the magnetic particles in the coated film, and in order to improve these properties, the magnetic particles to be dispersed in the vehicle are required to have as large an aspect ratio (major axial diameter/minor axial diameter) as possible, a uniform particle size distribution and no inclusion of dendrites.
As described above, magnetic iron oxide particles which have a substantially uniform particle size distribution, which are substantially free of dendtritess and which have a large aspect ratio (major axial diameter/minor axial diameter), are now in the strongest demand. In order to obtain magnetic iron oxide particles provided with these properties, it is necessary that as a starting material goethite particles have a substantially uniform particle size distribution, are substantially free of dendrites and have a large aspect ratio (major axial diameter/minor axial diameter).
Various attempts have been made to produce acicular goethite particles which have a uniform particle size distribution and are substantially free of dendrites. For example, Japanese Patent Application Laid-Open (KOKAI) No. 2-293330 (1990) discloses a method of producing acicular goethite particles by adding ascorbic acid or a salt thereof to the suspension containing ferrous hydroxide which is obtained by reacting an aqueous ferrous salt solution with not less than 1.0 equivalent of an aqueous alkali hydroxide and blowing an oxygen-containing gas into the suspension.
Magnetic iron oxide particles which have a uniform particle size distribution, are substantially free of dendtritess, and have a large aspect ratio (major axial diameter/minor axial diameter) and an excellent coercive force distribution are now in the strongest demand. The particles obtained by the method described in Japanese Patent Application Laid-Open (KOKAI) No. 2-293330 (1990) have an aspect ratio (major axial diameter/minor axial diameter) of at the most 10 to 11 and cannot be said to have a large aspect ratio (major axial diameter/minor axial diameter). The method includes the step of adding an alkaline earth metal in addition to an ascorbic acid or a salt thereof. But in this case, the aspect ratio (major axial diameter/minor axial diameter) is at the most 15. The acicular magnetic iron oxide particles produced from as a starting material these goethite particles have a uniform particle size distribution and are substantially free of dendrites, but the aspect ratio (major axial diameter/minor axial diameter) thereof is reduced to 1/2 to 1/3 after they are subjected to heat treatment.
Accordingly, it is an object of the present invention to provide acicular goethite particles which have a uniform particle size distribution, are substantially free of dendrites and have a large aspect ratio (major axial diameter/minor axial diameter) and acicular magnetic iron oxide particles further having excellent coercive force distribution in addition to the above-described properties.
As a result of studies undertaken by the present inventors, it has been found that by blowing an oxygen-containing gas into a ferrous salt reaction solution containing colloidal ferrous hydroxide or iron-containing colloidal precipitates which is obtained by reacting an aqueous ferrous salt solution with less than one equivalent of an aqueous alkali hydroxide solution, an aqueous alkali carbonate solution or an aqueous alkali hydroxide and alkali carbonate solution based on Fe.sup.2+ in the aqueous ferrous salt solution so as to oxidize the colloidal ferrous hydroxide or iron-containing colloidal precipitates and to produce acicular goethite particles through green rust, ascorbic acid or a salt thereof and, if necessary, a zinc compound being added to any of the solutions before the production of the acicular goethite particles, or by blowing an oxygen-containing gas into a ferrous salt reaction solution containing colloidal ferrous hydroxide or iron-containing colloidal precipitates which is obtained by reacting an aqueous ferrous salt solution with less than one equivalent of an aqueous alkali hydroxide solution, an aqueous alkali carbonate solution or an aqueous alkali hydroxide and alkali carbonate solution based on Fe.sup.2+ in the aqueous ferrous salt solution so as to oxidize the colloidal ferrous hydroxide or iron-containing colloidal precipitates and to produce acicular goethite nucleus particles through green rust, ascorbic acid or a salt thereof and, if necessary, a zinc compound being added to any of the solutions before the production of the acicular goethite nucleus particles; and growing the acicular goethite nucleus particles so as to produce acicular goethite particles, the thus-obtained acicular goethite particles and acicular magnetic iron oxide particles have a uniform particle size distribution, are substantially free of dendrites, and have a large aspect ratio (major axial diameter/minor axial diameter) and excellent coercive force distribution. On the basis of this finding, the present invention has been achieved.